Antimicrobial Agents & Antibiotics: A Guide to Fighting Germs

Antimicrobial agents are substances that kill or inhibit microorganisms. Antibiotics specifically target bacteria. Other classes: antivirals, antifungals, antiparasitics. Here are major antibacterial classes.

Major Antibiotic Classes & Mechanisms

Class	Mechanism	Key Examples	Typical Uses
Penicillins (β-lactam)	Inhibit cell wall synthesis (PBPs)	Penicillin G, amoxicillin, ampicillin, piperacillin-tazobactam	Strep, MSSA, syphilis, Listeria, enterococci
Cephalosporins (β-lactam)	Cell wall synthesis; β-lactamase stable (later gens)	1st: cefazolin; 3rd: ceftriaxone; 4th: cefepime; 5th: ceftaroline	Skin, UTI, pneumonia, meningitis, surgical prophylaxis
Carbapenems	Broadest β-lactam; resistant to most β-lactamases	Meropenem, imipenem, ertapenem	ESBL, CRE, severe sepsis, intra-abdominal
Aminoglycosides	30S ribosome → misread mRNA; bactericidal	Gentamicin, tobramycin, amikacin	Gram-negative sepsis, synergy for enterococci
Tetracyclines	30S ribosome → block tRNA; bacteriostatic	Doxycycline, minocycline, tigecycline	Atypical pneumonia, acne, rickettsial, chlamydia
Macrolides	50S ribosome → block translocation	Azithromycin, clarithromycin, erythromycin	CAP, pertussis, MAC, STI
Fluoroquinolones	Inhibit DNA gyrase & topoisomerase IV	Ciprofloxacin, levofloxacin, moxifloxacin	UTI, pneumonia, intra-abdominal, anthrax
Sulfonamides/Trimethoprim	Sequential folate synthesis blockade	TMP-SMX (Bactrim)	UTI, PCP, MRSA skin, nocardia
Glycopeptides	Bind D-Ala-D-Ala → block cell wall	Vancomycin	MRSA, C. diff (oral), enterococci
Oxazolidinones	50S → prevent initiation complex	Linezolid, tedizolid	VRE, MRSA pneumonia, skin
Lipopeptides	Calcium-dependent membrane depolarization	Daptomycin	MRSA bacteremia, skin (not pneumonia)

Critical Principles

• Culture before antibiotics when possible.
• Narrow spectrum targeted to likely pathogen.
• Right dose, route, duration.
• Stewardship: De-escalate, stop when not needed.
• Resistance is driven by misuse—preserve efficacy.